Power oscillations typically occur in large interconnected power systems, where two or more areas are interconnected through relatively weak alternating current (AC) transmission lines. These power oscillations are also referred to as interarea oscillations. Whether or not an oscillation is stable or unstable is a system property. Hence it is not the contingency initiating the oscillation that determines the necessary level of damping. Rather, the necessary level of damping depends on the state of the power system, i.e. operating condition, controller tuning, transmission lines, the generators in service, etc.
If a power oscillation between two areas of a power system is excited, the rotor angles of synchronous machines in one area will start to oscillate in counter phase with synchronous machines in the other area and thereby force a flow of active power back and forth between the areas. If the oscillation is sufficiently damped the oscillation will die out and the rotor angles return to a steady state. However, if there is insufficient damping in the power system for this particular oscillation an ever increasing amount of active power is exchanged between the two areas until other security devices, e.g. synchronous machine or transmission line safety equipment, trip the unit or component. A cascading effect of equipment tripping and ultimately a system black out can be the consequence.
In known electricity networks, the majority of the necessary damping torque is delivered by synchronous machines equipped with power system stabilizers (PSS). As it is known from practice, such stabilizing controllers are installed when a new power station is constructed. A PSS may also be retrofitted to existing power plants. A basic concept of a known PSS is to add an auxiliary signal to the voltage reference of the exciter and thereby modulate the excitation voltage of the synchronous machine to damp the power oscillations.
In view of the above described situation there exists a need for an improved technique that enables to damp power oscillations in an electricity network.